Sound velocities of aluminum‐bearing stishovite in the mantle transition zone

The elasticity of Al‐bearing stishovite with 1.0, 3.3, and 4.5 wt % Al 2 O 3 was investigated in the multianvil apparatus at high pressures and temperatures up to 21 GPa and 1700 K, by ultrasonic interferometry in conjunction with in situ X‐ray techniques. The moduli K S and G are found to decrease with increasing Al 2 O 3 content, while their pressure and temperature derivatives do not change in a significant manner for 1.0 and 3.3 wt % Al 2 O 3 . The temperature derivatives for 4.5 wt % Al 2 O 3 , however, are larger, which may result from a change in the Al substitution mechanism at high Al 2 O 3 content. It is shown that acoustic velocities of any mid‐ocean ridge basalt are lower by −0.4% than those calculated from pure stishovite data. Velocity perturbations up to −3.4% ( V P ) and −4.2% ( V S ) in subducted slabs are explained by the combination of the thermal equilibration (Δ T  ~ 600 K) of the slab and Al enrichment in stishovite.